There is known a blood-flow mapping display method that displays a blood-flow image in a superimposed manner on a tomographic image obtained by an ultrasound imager. A major flow mapping display method includes a color Doppler imaging that identifies a flowing direction and the magnitude of the blood flow by color, and a vector expression that expresses by arrows, or the like, the direction and the magnitude of the blood flow at plural points in the flow. The color Doppler imaging is a method that utilizes the Doppler effect to measure the blood-flow direction with respect to the direction of an ultrasound beam. By way of example, warm hues are assigned to the blood flow in the positive direction, whereas cold hues are assigned to the blood flow in the negative direction, thereby displaying color identification of the blood-flow directions.
However, the color Doppler imaging allows only direct measurement of a velocity component in the ultrasound beam direction, and it is not possible to display the flow direction, that is, in which direction the blood runs within the tomographic view. Considering this situation, it is suggested to estimate a velocity component being orthogonal to the ultrasound beam direction, using an equation of continuity of a two-dimensional flow, and the velocity of the tissue forming a boundary with the blood flow, and obtain a velocity vector from the velocity component being orthogonal to the ultrasound beam direction (Non Patent Document 1). However, actually, the flow is three dimensional, and it is unsure to what extent is reliable the velocity vector that is obtained according to the law of the two-dimensional flow.
The Patent Document 1 suggests a method to calculate a two-dimensional velocity vector by the color Doppler imaging, in order to display velocity variation along the flowing direction of the blood flow, and estimate a passage of the blood flow on the basis of thus calculated two-dimensional velocity vector. There is further disclosed a method that evaluates whether the estimation of the flow passage is proper or not, according to the integrity with nearby data points.
In this method, however, those plural data items nearby, serving as the evaluation standard, are also obtained following the law of the two-dimensional flow.
Therefore, it is unclear whether or not those are certain data items, and this leaves an underlying problem. In other words, even though the flow passage is estimated on the basis of the velocity vector that is evaluated as having high integrity with the nearby data points, the reliability of the flow passage is not necessarily high.